1. Field of the Invention
The present invention relates to a method of preparing polysilazane and a polysilazane solution which can minimize substitution of a silicon-hydrogen bond (hereinafter, an “Si—H bond”) of a starting compound using ammonia and, more particularly, to a method of synthesizing polysilazane using a solution which undergoes phase separation and a method of preparing a polysilazane solution for a wafer.
2. Description of the Related Art
Polysilazane, also referred to as polymeric silazane, is a polymer having a unit of —(SiR2—NR)— and is dissolved in an organic solvent for use. Polysilazane is transformed into silica or a silica material when heated, in which the silica material is used for electric and electronic applications, such as an insulating layer, owing to excellent insulating properties thereof.
Polysilazane can be prepared from a variety of starting compounds by various methods. Polysilazane obtained from ammonia is generally known as an ammonolysate, and one obtained from an amino compound containing an NH2 group except for ammonia is known as an aminolysate.
Polysilazane is prepared by a process of reacting chlorosilanes (SiCl4, HSiCl3 and H2SiCl2) with ammonia at high temperature in vapor phase conditions.
U.S. Pat. No. 4,395,460 discloses a method of preparing silazane through a pseudo-ammonolysis process, by which polysilazane is produced by introducing ammonia gas into a chlorodisilane solution dissolved in an inactive solvent.
Korean Patent Publication No. 2007-0040422 is directed to pseudo-ammonolysis products including novel silazane or polysilazane characterized by repeated units of Si—N in a polymeric compound having a reduced number of silicon-hydrogen (Si—H) bonds compared to the number of Si—H bonds in the starting compound. As for the pseudo-ammonolysis products having the Si—H bond, ammonium chloride (NH4Cl) generated during the reaction is dissolved in liquid anhydrous ammonia. The dissolved ammonium chloride acts as a catalyst to promote reaction between Si—H and NH3 to form an Si—NH2 bond. As a result, polysilazane with a reduced Si—H bond is obtained. However, the number of Si—H bonds is significantly reduced in polysilazane, thereby providing an excessive number of ammonia substituents. For polysilazane with the excessive ammonia substituents, an excessive amount of ammonia escapes, causing volume contraction, thereby causing cracks during a process of transforming the polysilazane into a dielectric material of SiO2 via coating a polysilazane solution on a wafer and annealing.